In the gypsum industry it has traditionally been the practice to crush or grind raw gypsum to a convenient size which is then calcined to produce calcium sulphate hemihydrate (or anhydrite), the dehydrated material being subsequently milled as desired to obtain a product having a particle size and other characteristics appropriate to the intended end use.
It has already been proposed to combine the calcination and grinding-of such materials in a single unit. For example, in U.S. Pat. No. 1,984,201 sub-divided gypsum is fed to the mill in a stream of hot gas, and the ground and calcined material is entrained in the hot gas together with a stream of cold gas. Fine calcined material leaves the unit entrained in the gas stream while coarser material falls back into the mill.
Techniques in which a so-called fluidized bed is employed are known from other industries. In British Patent GB-A-1193761 and in a European Patent Application published as EP-A-0 039 270, a granular material to be dried and/or heated and ground is fed through a pneumatic conduit by hot gases into a chamber containing a grinding device. The ground product is discharged from the top of the chamber by entrainment in the exhaust gases, and in the European application a restriction in the cross section of the upper part of the chamber (provided by a series of transverse bars) causes insufficiently ground material to fall back into the grinding zone.
In an earlier device described in British Patent GB-A-1080605 an agglutinant material such as clay is ground, sorted and dried in a fluidization reactor having a percussion grinder wholly or partly immersed in a fluidized bed of the material. In this apparatus, the base of the reactor is perforated in the classical manner of a fluidized bed reactor, and the hot gases entering the reactor through the perforated base not only fluidize the material in the bed but also carry off the product from the top of the reactor.
In these and similar systems the material, even if it forms a definable bed at all, is present in a relatively low density mixture of gas and solids (i.e. a low solid to gas ratio), and the solid to gas ratio is even lower in the upper region of the chamber where the ground material is being exhausted with the gases. Such systems require large solids-gas separators to recover the ground material from the gas stream.
We have invented various systems for the continuous calcination of gypsum in which a bed of gypsum is maintained in fluidised condition at least in part by the water vapour evolved during dehydration, see for example GB 1,018,464, 1,488,665 and 2,043,319. Such systems have greater thermal efficiency than conventional calcination kettles, have reduced maintenance costs and are of lower capital cost. They have been widely adopted in the industry. A significant difference between such fluidised bed systems and the gas entrainment systems mentioned above is that they employ a relatively dense bed (i.e. with high solid to gas ratio) which can be easily discharged under gravity, for example by an overflow or weir. By comparison, the relatively low density streams of gas and solids mentioned earlier involve reduced efficiency and higher costs. Techniques suitable for use with a low solids content gas stream are different from those used in a solids-rich fluidised bed and would not be expected to work in the different situation.
We have now found surprisingly that a rotary grinding or milling machine can function satisfactorily in the bottom of a relatively dense fluidised bed, despite the density of the bed and the very substantial back pressure such a bed exerts in the grinding region. This discovery has enabled us to develop a heating and grinding system having significant potential advantages over conventional equipment for separate calcining and grinding or previously available gas-entrained heating and grinding equipment.